This application claims priority to an application entitled xe2x80x9cRadio Frequency Filter of Combline Structure Having Frequency Cut-off Circuit and Method for Implementing The Samexe2x80x9d, filed in the Korean Industrial Property Office on Feb. 26, 2001 and assigned Serial No. 2001-9656, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a radio frequency filter using transmission lines, and particularly to a radio frequency filter for cutting off a specific frequency from a given filtering frequency band, and a method for implementing such a radio frequency filter.
2. Description of the Related Art
In the field of portable communication equipment such as portable telephones, size and manufacturing cost are of great concern. Such concerns do not apply solely to portable communication equipment. Various techniques to address these concerns have been actively developed.
One method for achieving a reduction in size is to use a configuration, which can be implemented in a limited space, such as transmission lines (striplines or micro striplines), in place of a passive element occupying a large area. A representative example of such a configuration may be a filter implemented using transmission lines (striplines or micro striplines) to have a filter function for extracting signals of a desired frequency band while cutting off noise signals of other frequency bands. Such a filter may be used in various fields including radio communication systems. In radio communication systems, the filter can be used for a receiver to receive desired signals or for a transmitter to transmit desired signals.
An example of a conventional stripline filter is disclosed in U.S. Pat. No. 4,963,843 issued to Motorola, Inc. on Oct. 16, 1990. Now, a conventional combline stripline filter will be described in brief, with reference to the disclosure of U.S. Pat. No. 4,963,843.
The conventional combline stripline filter is designed using conductive strips each having one end connected to the ground and the other end capacitively loaded to the ground. That is, the combline stripline filter includes a substrate having top and bottom surfaces each forming a ground plane. An inner circuitry layer is formed between the top and bottom surfaces of the substrate. The combline stripline filter also includes a ground area having a plurality of angled edges coupled to the ground planes. The inner circuitry layer is formed by combline resonators each coupled to the ground at one end thereof and capacitively loaded to the ground at the other end thereof. This combline stripline filter uses pattern capacitors in that the combline resonators are arranged in an interlayered fashion.
However, such a stripline filter, which uses pattern capacitors having the above mentioned structure, has problems of increased layout size and an increased error rate occurring in the pattern capacitors due to interference. Furthermore, it is difficult to connect the stripline filter to other devices. Where the pattern capacitors are capacitively loaded to the ground, it is difficult to accurately calculate the loaded capacitance. Since the capacitance between each pattern capacitor and the ground may be varied depending on the material of the substrate, the initial manufacture of the stripline filter may be difficult. Furthermore, this stripline filter is restricted in terms of its size and position when it is connected to other devices. This is because the connection of each pattern capacitor to an input/output pad and the ground is made at ends of the substrate.
Similar to a general filter using a passive element, the above mentioned filter using transmission lines has a desired frequency bandwidth for its filtering operation. The frequency bandwidth is determined by the space between adjacent transmission lines, the width of each transmission line, and the capacitance of the pattern capacitors coupled to the transmission lines.
In pace with recent developments in the communication industries, more sub-divided frequency bands have been used. However, this causes a reduction in the width between adjacent allocated frequency bands. As a result, the allocated frequency bands may interfere with each other. For this reason, it may be impossible to provide radio services of a good quality. Where the filter uses a reduced frequency bandwidth in order to reduce the interference between the allocated frequency bands, another problem of a reduction in the gain of the filter occurs even though the interference is reduced.
Therefore, an object of the invention is to solve the above mentioned problems, and to provide a radio frequency filter using transmission lines, which includes a frequency cut-off circuit arranged at a specific position of the filter and adapted to cut off a specific frequency from a frequency band having a given frequency bandwidth, and a method for implementing the radio frequency filter.
Another object of the invention is to provide a radio frequency filter capable of cutting off a specific frequency from a given frequency band, using inductance and capacitance, and a method for implementing the radio frequency filter.
Another object of the invention is to provide a radio frequency filter capable of cutting off a specific frequency by fixing its inductance at a specific frequency band while varying its capacitance, and a method for implementing the radio frequency filter.
In accordance with one aspect, the present invention provides, in a radio frequency filter of a combline structure including an input terminal, an output terminal, transmission lines arranged in a pair, each of the transmission lines having a desired width while being connected to a capacitance compensating circuit through a via hole, whereby the radio frequency filter has a predetermined frequency bandwidth, a frequency cut-off circuit for cutting off a specific frequency from a frequency band having the predetermined frequency bandwidth, the frequency cut-off circuit comprising:
an inductive transmission line extending from the output terminal by a length determined to provide an approximate inductance corresponding to a calculated value approximate to an inductance for obtaining the specific frequency; and
a capacitive element coupled to the approximate inductance provided by the inductive transmission line, so that it has a capacitance for obtaining the specific frequency;
wherein the inductive transmission line is connected to the capacitive element through a via hole formed at an end of the transmission line opposite to the output terminal, from which the transmission line extends.
In accordance with another aspect, the present invention provides, in a radio frequency filter of a combline structure including an input terminal, an output terminal, transmission lines arranged in a pair, each of the transmission lines having a desired width while being connected to a capacitance compensating circuit through a via hole, respectively, whereby the radio frequency filter has a predetermined frequency bandwidth, a method for cutting off a specific frequency from a frequency band having the predetermined frequency bandwidth, comprising the steps of:
calculating an inductance approximate to an inductance for obtaining the specific frequency;
determining a length of the inductive transmission line to extend from the output terminal, based on the approximate inductance;
calculating a capacitance of a capacitive element coupled to the approximate inductance provided by the inductive transmission line while being connected to the inductive transmission line through a via hole to obtain the specific frequency.